1. Field of the Invention
This invention relates to a surface plasmon sensor for quantitatively analyzing a substance in a sample by utilizing the occurrence of surface plasmon. This invention also relates to an evanescent ellipsosensor, wherein a light beam impinging upon a prism is totally reflected from an interface between the prism and a sample, a change in condition of polarization of the light beam due to the total reflection is detected, and a substance in the sample is thereby analyzed. This invention further relates to an electrophoresis sensor for analyzing a substance in a sample by utilizing electrophoresis.
2. Description of the Prior Art
In metals, free electrons vibrate collectively, and a compression wave referred to as a plasma wave is thereby produced. The compression wave occurring on the metal surface and having been quantized is referred to as the surface plasmon.
Various surface plasmon sensors for quantitatively analyzing a substance in a sample by utilizing a phenomenon, in which the surface plasmon is excited by a light wave, have heretofore been proposed. As one of well known surface plasmon sensors, a surface plasmon sensor utilizing a system referred to as the Kretschman arrangement may be mentioned. The surface plasmon sensor utilizing the system referred to as the Kretschman arrangement is described in, for example, Japanese Unexamined Patent Publication No. 6(1994)-167443.
Basically, the surface plasmon sensor utilizing the system referred to as the Kretschman arrangement comprises (i) a prism, (ii) a metal film, which is formed on one surface of the prism and is brought into contact with a sample, (iii) a light source for producing a light beam, (iv) an optical system for causing the light beam to pass through the prism and to impinge upon the interface between the prism and the metal film such that various different angles of incidence may be obtained with respect to the interface, and (v) a photo detecting means capable of detecting the intensity of the light beam, which has been totally reflected from the interface, with respect to each of the various different angles of incidence.
In order for various different angles of incidence to be obtained, a light beam having a comparatively small beam diameter may be deflected and caused to impinge upon the interface. Alternatively, a light beam having a comparatively large beam diameter may be converged on the interface such that the light beam may contain components, which impinge at various different angles of incidence upon the interface. In the former case, the light beam, which comes from the interface at various different angles of exit in accordance with the deflection of the incident light beam, may be detected with a small photodetector, which moves in synchronization with the deflection of the light beam, or may be detected with an area sensor extending in the direction, along which the angle of exit of the light beam changes. In the latter case, the light beam may be detected with an area sensor extending in a direction such that the area sensor can receive all of the light beam components coming from the interface at various different angles of exit.
With the surface plasmon sensor having the aforesaid constitution, when a light beam composed of a P-polarized light component (i.e., a polarized light component normal to the reflection interface) impinges at a specific angle of incidence .theta..sub.SP, which is not smaller than the total reflection angle, upon the metal film, an evanescent wave having an electric field distribution occurs in the sample, which is in contact with the metal film, and the surface plasmon is excited at the interface between the metal film and the sample by the evanescent wave. In cases where the wave vector of the evanescent wave coincides with the wave number of the surface plasmon and wave number matching is obtained, the evanescent wave and the surface plasmon resonate, and energy of the light transfers to the surface plasmon. As a result, the intensity of the light, which is totally reflected from the interface between the prism and the metal film, becomes markedly low.
If the wave number of the surface plasmon is found from the specific angle of incidence .theta..sub.SP, at which the aforesaid phenomenon occurs, a dielectric constant of the sample can be calculated. Specifically, the formula shown below obtains. ##EQU1## wherein K.sub.SP represents the wave number of the surf ace plasmon, .omega. represents the angular frequency of the surface plasmon, c represents the light velocity in a vacuum, .di-elect cons..sub.m represents the dielectric constant of the metal, and .di-elect cons..sub.s represents the dielectric constant of the sample.
If the dielectric constant .di-elect cons..sub.s of the sample is found, the concentration of a specific substance contained in the sample can be calculated from a predetermined calibration curve, or the like. Therefore, the specific substance contained in the sample can be quantitatively analyzed by finding the specific angle of incidence .theta..sub.SP, at which the intensity of the reflected light beam becomes low.
However, with the conventional surface plasmon sensors described above, the problems are encountered in that, when the substance contained in a trace amount in the liquid sample is analyzed, the sensitivity with which the substance to be analyzed is detected cannot be kept high, and a long time is required to carry out the analysis.
Also, it has heretofore been known that, when a light beam traveling in a first medium is totally reflected by an interface between the first medium and a second medium, which has a refractive index lower than the refractive index of the first medium, light referred to as the evanescent wave leaks to the second medium. When the light beam impinges upon the interface, the electric field of light changes in phase before the light beam is totally reflected from the interface and after the light beam is totally reflected from the interface. The change in phase varies for the P-polarized light component (normal to the reflection interface) and the S-polarized light component (parallel to the reflection interface). The change in the condition of polarization is inherent in accordance with the second medium which interacts with the evanescent wave.
Evanescent ellipsosensors for analyzing a substance contained in a sample by utilizing the phenomenon described above have heretofore been used. In the evanescent ellipsosensors, a constitution for totally reflecting a light beam from an interface between the sample and a prism is employed, and a technique (ellipsometry) for detecting a change in phase difference, i.e. a change in condition of polarization, is applied to the constitution. Such an evanescent ellipsosensor is described in, for example, PHYSICAL REVIEW LETTERS, Vol. 57, No. 24, 15 December, 1986, pp. 3065-3068. With the evanescent ellipsosensors, the prism is employed as the aforesaid first medium, the sample serving as the aforesaid second medium is brought into close contact with one surface of the prism, and the light beam is totally reflected from the interface between the prism and the sample. A change in condition of polarization due to the total reflection is detected, and physical properties or a total amount of the substance in the sample is thereby determined.
However, with the conventional evanescent ellipsosensors described above, the problems are encountered in that, when the substance contained in a trace amount in the liquid sample is analyzed, the sensitivity with which the substance to be analyzed is detected cannot be kept high, and a long time is required to carry out the analysis.
Further, electrophoresis apparatuses for analyzing substances contained in a sample by utilizing electrophoresis have heretofore been used. Such an electrophoresis apparatus is described in, for example, Japanese Unexamined Patent Publication No. 62(1987)-220853. Basically, with the electrophoresis apparatuses, a DC voltage is applied across an electrophoresis medium having been impregnated with a sample, and substances having electric charges, such as protein, protein decomposition products, nucleic acid, and nucleic acid decomposition products, which substances are contained in the sample, are thereby caused to migrate through the electrophoresis medium. The substances are thus separated spatially in the electrophoresis medium by the utilization of differences in migration speed among the substances.
In the conventional electrophoresis apparatuses, ordinarily, gel sheets constituted of a polyacrylamide gel, an agarose gel, or the like, are employed as the electrophoresis media. Also, ordinarily, in order for the spatially separated substances to be analyzed, techniques are employed wherein the substances in the sample are labeled with fluorescent substances, radioactive isotopes, and the like, and the positions of the labeled substances after migrating through the electrophoresis medium are recorded on a photographic material, a stimulable phosphor sheet described in Japanese Unexamined Patent Publication No. 62(1987)-90600, or the like.
However, in order for the positions of the labeled substances to be recorded, it is necessary to carry out exposure of the photographic material, the stimulable phosphor sheet, or the like, development of a latent image on the photographic material, a process for reading out image information having been recorded on the stimulable phosphor sheet, and the like. Therefore, with the conventional electrophoresis apparatus, considerable time and labor are required to analyze the samples. Also, considerable time and labor are required to label the substances in the sample with radioactive isotopes, or the like. Further, there is the risk that the labeling operation adversely affect the human body.